What Is Oil? Essay, Research Paper

There are three essentials in the creation of a crude oil field:

First, a “source rock” whose geologic history allowed the formation of crude oil. This usually is a fine-grained shale rich in organic matter.

Second, migration of the oil from the source rock to a “reservoir rock,” usually a sandstone or limestone that’s thick and porous enough to hold a

sizable accumulation of oil. A reservoir rock that’s only a few feet thick may be commercially producible if it’s at a relatively shallow depth and near

other fields. However, to warrant the cost of producing in more challenging regions (the Arctic North Slope, for example) the reservoir may have to

be several hundred feet thick.

Third, entrapment. The earth is constantly creating irregular geologic structures through both sudden and gradual movements – earthquakes, volcanic

eruptions and erosion caused by wind and water. Uplifted rock, for example, can result in domelike structures or arched folds called anticlines.

These often serve as receptacles for hydrocarbons. The probability of discovering oil is greatest when such structures are formed near a source

rock. In addition, an overlying, impermeable rock must be present to seal the migrating oil in the structure.

The oldest oil-bearing rocks date back more than 600 million years; the youngest, about 1 million. However, most oil fields have been found in rocks

between 10 million and 270 million years old.

Subsurface temperature, which increases with depth, is a critical factor in the creation of oil. Petroleum hydrocarbons rarely are formed at temperatures

less than 150 degrees Fahrenheit and generally are carbonized and destroyed at temperatures greater than 500 degrees. Most hydrocarbons are found at

“moderate” temperatures ranging from 225 to 350 degrees.